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Full text of "Text Book Of Mechanical Engineering"

Marker-off's  Tools.                               185

shewn at A, Fig. 192; and Cubical blocks are also provided,
of several sizes, but each of known depth and so figured. They
have their surfaces truly parallel, and are used to gain greater
height for the Scribing block, as well as for the purpose of
packing up the work (see B, "Fig. 192).

The Scribing Block, Fig. 193, is a most important tool.
It consists of an upright pillar A, fixed in a base B, which has
been truly scraped underneath. Upon A slides the head D,
which can be set to any height by tightening the nut H, a pointer
or scriber E being at the same time fixed at any convenient angle
by nut G. Most scribing blocks have no other adjustment, but
in that shewn there is a screw at F for further accuracy; here
the head c is first clamped, and D left free until finally adjusted
by the screw F, after which D is firmly tightened and the scribing
clone. The scriber has one point straight and the other curved,
the uses of these being shewn, where j can be made to 'scribe' a
horizontal line on the work by moving the block along the table,
and H may serve to ' feel? the height of certain other work. The
scriber is of steel, well-hardened, and must be kept sharp by
rubbing on an oilstone.

The Hand Scriber (Fig. 194) is to the marker-off what the
pencil is to the draughtsman. It is pointed at one end, and
hooked at the other for hanging to the pocket.

Compasses and Trammels must be provided for striking
^                   arcs of various radii, and as some pressure is required to make a

i                     sufficiently clear line on the work, both these tools should be

sufficiently rigid; the former being supplied, for this purpose,
with an arc and screw. Both tools are shewn at Fig. 195.

Accurate measuring Rules, with inches divided into eighths

and tenths; Squares large and small (3 in. to 3 ft); Straight

Edges of different lengths;  and Callipers, both for internal

|f                  , and external measurement, are all necessary tools; while if the

ir                    work is too large to mark-off on arable it should be levelled, and

'fi                   all lines be drawn by reference to an ideal horizontal or vertical

^                    plane,  necessitating the  use of either a Spirit Level or the

I                    Square and Plumb-Bob shewn at Fig. 196, the latter being

| /               the only tool in favour with the best workmen, as levels are

I                   known to get out of order so easily.